1. Field of the Invention
The present invention relates to processes for the conversion and safe disposal of waste materials containing soluble arsenic salts.
2. Description of the Prior Art
Many companies in the United States use arsenical materials in various products and processes, and it is important to have available a disposal capability for the by-products or wastes thus generated which will not degenerate the natural environment.
Since arsenic is an element and cannot be destroyed, the best procedure for treating arsenical waste materials is to detoxify the same by changing their chemical structure to a non-soluble state, which is well suited to secure landfill disposal. The process of the present invention accomplishes that goal.
Present process for treating arsenical materials are extremely sophisticated, generally comprising two or three different process stages. They are also extremely costly and are generally designed to be point source specific--that is, they are designed to treat a specific waste material under rigid parameters.
The process of the present invention permits a great deal of latitude in process parameters and permits the process of detoxification to be completed in a single step operation.
The process of the present invention is unique as compared to prior art processes, and is much more cost effective because it utilizes a very simple equipment package and, most importantly, in a highly preferred embodiment, it utilizes three waste products which, in effect, have no commercial value and, therefore, almost eliminates the cost of process chemicals to provide an end product that is well suited environmentally for secure landfill disposal.
The process of the present invention is also superior to prior art processes because it directly addresses treatment of arsenical waste for purposes of detoxification prior to disposal, and the simplicity of the equipment involved and the availability of the component waste required for the process makes it readily duplicatable in substantially all areas of the United States.
U.S. Pat. No. 3,911,078, Nadkarni, et al. discloses a method for removing arsenic and antimony from copper ore concentrates. In the described process, arsenic, along with any antimony present, is removed from copper ore concentrates by leaching the ore with an aqueous solution of sodium sulfide containing sodium hydroxide. Copper sulfide, which remains undissolved and in the form of particles, is removed by filtering to leave a pregnant solution containing dissolved sodium thioarsenate and sodium thioantimonate, which are then converted to the sulfides or to ferric arsenate and ferric antimonate by any of several reaction routes--e.g., acidification with a strong mineral acid, such as sulfuric acid, to produce arsenic sulfide which is, in turn, converted to ferric arsenate by acid pressure leaching, partial neutralization by lime and the addition of ferric sulfate.
The process described in Nadkarni, et al. is extremely complicated, requires multiple steps and is directed to the specialized art of treating copper ore concentrates.
U.S. Pat. No. 3,107,977, Borvali, et al., relates to a process for treating arseniureted ores, particularly of cobalt and/or nickel. The process comprises placing the arseniureted ores in suspension in sulfuric acid and progressively adding nitric acid to oxidize the arsenic present. Excess sulfuric acid is utilized to transform most of the metals present into sulfates, and addition of nitric acid is controlled to promote a limited oxidation of the arsenides in the ore. A limited oxidation prepares the system for the subsequent separation, in the form of As.sub.2 O.sub.3, of any excess arsenic present which is not necessary to maintain a weight ratio of iron to arsenic near one, and which excess thus will not be necessary in the subsequent formation of ferric arsenate. Following a complicated series of crystallizations, dissolutions, filtrations and chlorine treatments, calcium carbonate is finally added to precipitate arsenic and iron as ferric arsenate using a 1:1 weight ratio between arsenic and iron.
Borvali, et al. is similar to Nadkarni, et al. in the sense of involving an extremely complicated processing sequence and in relating to the treatment of ores per se.
U.S. Pat. No. 1,509,688, Parsons, et al., relates to a method of recovering arsenic from oxidized metallurgical dusts or fumes, wherein the dusts or fumes are leached with hot water and, in certain instances, with a caustic soda solution of sufficient alkalinity to correspond to about a disodium compound of the arsenic to be extracted, thereby yielding a substantially neutral extract.
U.S. Pat. No. 1,952,290, Schopper, discloses a process for recovering arsenic, in the form of arsenious acid, from arseniferous metallurigal intermediate products and by-products. According to the described process, arsenic in the metallurgical intermediate products and byproducts is separated in the form of arsenic trioxide by first converting the arsenic present into arsenic acid, for example, by roasting, dissolving the arsenic acid by extracting the roasted product with a suitable acid and thereafter reducing the same to arsenic trioxide by treatment with a reducing agent. Suitable acids include sulfuric acid, and, when arsenic is initially present in the pentavalent form, leaching with an acid such as sulfuric acid can be effect without any previous oxidation treatment.
U.S. Pat. No. 863,940, Baschen, et al., discloses a process for purifying arsenical sulfuric acid to obtain arsenic therefrom in the form of liquid arsenious chloride which comprises contacting the arsenical sulfuric acid with hydrochloric acid gas at an elevated temperature.
U.S. Pat. No. 3,684,492, Colombini, et al., describe a process for preparing arsenic-free copper cement from arsenical acid solutions obtained by chlorinating roasting or by chlorinating volatilization of iron minerals (pyrite ashes) which consists of:
Mixing the starting solution with ferrous salt;
Bringing the resulting solution to a pH between 3 and 3.5 at a temperature between 20.degree. and 80.degree. C. using a neutralizer, followed by filtering to obtain a filtrate; and PA1 Cementing the filtrate with excess metal iron at 20.degree.-40.degree. C. to obtain copper cement free of arsenic and a discharge solution free of arsenic. PA1 Smelting a charge of the above material (ore or ore concentrate) in combination with calcium values, a sodium salt and a reducing agent to convert arsenic to a water-insoluble calcium compound of arsenic and oxygen; PA1 Leaching an alkaline matte obtained from the smelting with an aqueous liquid; and PA1 Separating the aqueous liquid leach solution containing the desired materials from an insoluble solid leach residue containing the insoluble calcium compound of arsenic and oxygen. PA1 1-5% monosodium methane arsenate PA1 1-5% cacodylic acid PA1 1-5% sodium arsenite PA1 5-80% inorganic chloride and sulfate salts, typically sodium chloride and sodium sulfate PA1 0-100% moisture. PA1 Sulfuric acid; PA1 Ferrous ions to insure proper setting of the reaction system to solid form and sufficient iron in solution (either in ferrous and/or ferric form) to react with arsenic derived from the water-soluble arsenic salts present.* FNT *Note: Although ferric sulfate can be used as a substitute in the reaction, ferric sulfate will not cause the end product to "set-up" as will ferrous sulfate. Therefore, in regard to the scope of the invention, we will refer strictly to the ferrous state of iron in that it is most desirable in relation to the physical state of the end product. PA1 Approximately 1% magnetic iron oxide (Fe.sub.3 O.sub.4); PA1 About 10-12% calcium carbonate; PA1 About 2% insoluble silicates; PA1 Trace heavy metals.
Arsenic is precipated in the form of Fe arsenite in the above process.
U.S. Pat. No. 3,687,628, Nelen, et al., discloses a process for purifying sulfur from arsenic wherein molten sulfur is contacted with calcium oxide or calcium hydroxide in the presence of an aqueous solution of inorganic salts whose boiling point is higher than the melting point of sulfur and whose anions form soluble compounds with calcium cations.
U.S. Pat. No. 3,969,202, Albrethsen, et al., disclose a process for recovering antimony values from ores containing sulfo-antimony compounds of copper and arsenic which comprises: